This invention relates to coating materials and methods for producing a multilayer finish, particularly a multilayer two-tone, chip resistant finish, which has improved durability, on a substrate such as an automobile or truck.
Transportation vehicles, such as automobile and truck bodies, are treated with multiple layers of coatings which enhance the appearance of the vehicle and also provide protection from corrosion, scratch, chipping, ultraviolet light, acid rain and other environmental conditions. Basecoat/clearcoat finishes for automobiles and trucks have been commonly used over the past two decades, in a “wet-on-wet” application, i.e., the clear coat is applied before the base coat is completely cured. In typical fashion, the basecoat/clearcoat finish is typically applied over a previously cured primer surfacer coated substrate. It is also common to apply a special chip resistant primer in the low body areas of automobile and truck bodies, during the primer surfacer application stage.
The desire for even more unique and attractive color styling has led the automobile and truck Original Equipment Manufacturers (OEM) to produce vehicles with multiple colored, or “two-toned,” finishes. A typical procedure used to produce a chip resistant “two-tone” finish on a vehicle substrate involves the following:                I) Application of a lower body chip resistant primer over an electrocoated vehicle substrate;        II) Application of a primer surfacer to the entire substrate;        III) Bake curing the prime coated substrate;        IV) Applying a main body color, which is typically a waterborne basecoat to the vehicle substrate;        V) Applying clearcoat over the main color basecoat;        VI) Bake curing and covering with a protective membrane, the upper body main color basecoat/clearcoat finish area of the substrate;        VII) Applying accent color, which is typically a solventborne basecoat, in accent area        VIII) Applying accent clear coat in accent area, which is typically the same clear coat as used in step (V) above        IX) Bake curing the accent basecoat/clearcoat finish, and removing the protective membrane.        
One disadvantage with such a process is that the clearcoats in use nowadays experience compatibility problems with a variety of basecoat formulations. Most clearcoats do not have good appearance and adhesion to both waterborne and solventborne basecoats. Commonly used waterborne basecoats for the main body portion, particularly those containing free amines, often appear to cause unacceptable wrinkling and poor appearance in subsequently applied and cured clearcoat formulations. It has been found that a clearcoat composition containing polymeric high imino melamine can provide good appearance and wrinkle resistance over amine containing waterborne basecoats. However, it has also been found that polymeric high imino aminoplast resins can lead to poor scratch and mar resistance and unacceptable adhesion for over baked solvent or waterborne basecoats, which are now more popularly practiced in the automotive assembly plants where basecoats need to be sprayed, such as in two-tone operations, over a wet primer in the primer spray booth and baked in the former primer only ovens. For a successful two-tone process, the clearcoat composition must be compatible with both waterborne and solventborne basecoats and provide acceptable levels of appearance and durable adhesion to the underlying basecoat. Furthermore, the auto plants are now trying to elevate the two-tone process from the lower body of the truck to the mid- or high-line of the vertical surface. Durability of finishes for adhesion performance becomes even more critical.
Therefore, there is a need for a coating composition and application methods which provide multiple colored two-tone finishes having improved durability and adhesion over baked substrates without sacrificing wrinkle resistance over waterborne basecoats. There is also a desire to carry out this method in a minimum number of coating steps and bake curing cycles.